The invention relates to a telecommunication system comprising a plurality of mutually coupled switching centers and at least one coupled control unit.
The invention further relates to a control unit for a telecommunication system comprising a plurality of mutually coupled switching centers and a switching center for a telecommunication system comprising a plurality of mutually coupled switching centers and at least one control unit.
A switching center with an LCR function is known, for example, from U.S. Pat. No. 5,452,351 which function may be coupled to other switching centers in a telecommunication system. The LCR (Least Cost Routing) function as a criterion for the path assignment is a known feature which is rendered available especially in private telecommunication systems having a plurality of coupled switching centers. When there is a connection request from a user terminal, the associated switching center renders the path available to the desired user terminal that causes the least operating cost. These preferred paths are computed beforehand based on characteristic data (for example, charges per time unit, length of the time unit, instant of the connection request etc.) and stored in suitable form. Also alternative paths having respective priorities may be computed which paths are then selected when the optimum path cannot be used for the moment (for example, because of overload or interruption).
A plurality of user terminals are assigned to the switching center disclosed in U.S. Pat. No. 5,452,351. In addition, a control unit which includes storage means and processing circuits for LCR functions is assigned to the switching center.
A terminal which has a keyboard for inputting data and an LCD display for outputting data is connected to the control unit. The terminal supplies the control unit with LCR data which are stored in the storage means. When a request for a connection from one of the terminals of the private telecommunication system to an external user is generated, the control unit determines the path to the called other user terminal based on the LCR data stored in the control unit, which path causes the least operating cost per time unit.
LCR data is stored in tables in the storage means. Codes which are dialled when there is a request for a connection to an external user and which determine the path to the external user that involves the lowest operating cost are stored under specific addresses. The beginning and end of the path are then determined by the directory numbers of the called internal user and of the called external user.
When a path is assigned from the LCR point of view, it is highly advantageous to use one's own private telecommunication system for the greater part of the path. As a result, call transfers take place first between the coupled switching centers which contain their own switching tables with LCR data. For an incoming call, the respective switching center recognizes only the beginning and the end of the path from which the most favorable path for the further switching is determined by means of the LCR data. If this path cannot be used as a result of a current overload or interruption, the switching tables contain an alternative path. Since the addressed switching center does not know the previous route of the call, the call is permanently sent to and fro between two switching centers, so that an endless loop evolves. For that case, an additional feature is included in the network protocol, according to which all the calls that have passed through more than a certain number of switching centers are rigorously erased. The disadvantages resulting from this method, and also the time loss until the erasure, are obvious.
Naturally, in addition to a path assignment while least cost routing (LCR) is taken into account, it is also possible to establish the switching tables based on other points of view such as, for example, paths or transmission rate determined by the optimum load of the transmission capacity.